Hitherto, as disclosed in Japanese Unexamined Patent Publication No. 2011-213295, a device that restrains unreasonable passing of a host vehicle when a relative speed between a preceding vehicle and the host vehicle during the host vehicle coming close to the preceding vehicle is small, is known.
When the relative speed is smaller than a predetermined value, the device disclosed in Japanese Unexamined Patent Publication No. 2011-213295 adjusts the speed of the host vehicle so as to conform to the speed of the preceding vehicle. In addition, when the relative speed is equal to or greater than a predetermined value, and it is predicted that a following vehicle traveling at the rear of the host vehicle before the host vehicle completes passing of the preceding vehicle catches up with the host vehicle, the device disclosed in Japanese Unexamined Patent Publication No. 2011-213295 adjusts the speed of the host vehicle so as to conform to the speed of the preceding vehicle.